Carbon based electrocatalysts for fuel cells

ABSTRACT

Novel proton exchange membrane fuel cells and direct methanol fuel cells with nanostructured components are configured with higher precious metal utilization rate at the electrodes, higher power density, and lower cost. To form a catalyst, platinum or platinum-ruthenium nanoparticles are deposited onto carbon-based materials, for example, single-walled, dual-walled, multi-walled and cup-stacked carbon nanotubes. The deposition process includes an ethylene glycol reduction method. Aligned arrays of these carbon nanomaterials are prepared by filtering the nanomaterials with ethanol. A membrane electrode assembly is formed by sandwiching the catalyst between a proton exchange membrane and a diffusion layer that form a first electrode. The second electrode may be formed using a conventional catalyst. The several layers of the MEA are hot pressed to form an integrated unit. Proton exchange membrane fuel cells and direct methanol fuel cells are developed by stacking the membrane electrode assemblies in a conventional manner.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/661,770 filed Mar. 15, 2005; U.S. Provisional Application Ser.No. 60/662,284 filed Mar. 16, 2005; and U.S. Provisional ApplicationSer. No. 60/735,777 filed Nov. 12, 2005; the contents of each of whichare hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to improved electrodes for a membrane electrodeassembly (MEA) for use in proton exchange membrane fuel cells (PEMFC)and direct methanol fuel cells (DMFC), and more particularly a method ofmanufacturing platinum (Pt) and platinum-ruthenium (PtRu) based membraneelectrode assemblies using a filtration process incorporating carbonnanomaterials, such as carbon nanotubes.

A fuel cell is a device that converts the chemical energy of a fuel andan oxidant directly into electricity without combustion. The principalcomponents of a fuel cell include electrodes catalytically activated forthe fuel (anode) and the oxidant (cathode), and an electrolyte toconduct ions between the two electrodes, thereby producing electricity.The fuel typically is hydrogen or methanol, and the oxidant typically isoxygen or air.

Fuel cells are electrochemical devices that convert chemical energydirectly into electrical energy. Compared with internal combustionengines, fuel cells are not limited by the Carnot cycle and in principlecould have higher efficiency. With pure hydrogen as the fuel, fuel cellsare very environmentally friendly. The combination of high efficiency,low environmental impact, and high power density has been and willcontinue to be the driving force for vigorous research in this area fora wide variety of applications such as transportation, residential powergeneration, and portable electronic applications. For portableelectronic applications, important features include high power density(i.e., longer battery life) and compactness.

Silicon-based microfabrication technology is amongst the promisingapproaches for fabrication of compact micro fuel cells. However, thecurrent methods for making electrodes for fuel cells, which typicallyincludes spraying and/or brushing of platinum supported on carbonpowder, is incompatible with microfabrication techniques. Therefore,there is need for improved electrodes and methods of preparing suchelectrodes for PEMFCs and DMFCs.

Direct methanol fuel cells (DMFCs) have attracted enormous attention asa promising power source for portable electronics applications such aslaptop computers and cell phones. The interest in commercializing DMFCsis in part due to the fuel cell's simple system design, high energydensity and the relative ease with which methanol may be transported andstored, as compared with hydrogen. In the state-of-the-art DMFCs,platinum supported on a carbon substrate is configured in the cathode asa catalyst for activating the oxygen reduction reaction (ORR). Aplatinum-ruthenium alloy is usually used as the anode electrocatalyst,and may be supported on a carbon substrate. The electrolyte is usually aperfluorosulfonate membrane, for which NAFION (available from DuPont) isa commonly utilized commercially available membrane. One of the majorproblems encountered in DMFCs is methanol crossover from the anode tothe cathode. The permeated methanol causes “poisoning” of the cathodeplatinum catalyst and depolarization losses due to the simultaneousoxygen reduction and methanol oxidation on the platinum catalyst.

Reference is made herein to the well-known rotating disk electrode,which is used in the testing of the present invention as describedbelow. As will be appreciated by those of ordinary skill in the art, therotating disk electrode (RDE) consists of a disk on the end of aninsulated shaft that is rotated at a controlled angular velocity.Providing the flow is laminar over all of the disk, the mathematicaldescription of the flow is surprisingly simple, with the solutionvelocity towards the disk being a function of the distance from thesurface, but independent of the radial position. The rotating diskelectrode is used for studying electrochemical kinetics underconditions, such as those of testing the present invention, when theelectrochemical electron transfer process is a limiting step rather thanthe diffusion process.

Polymer electrolyte based low temperature fuel cells, with their twobest known variants, proton exchange membrane fuel cells (PEMFC) anddirect methanol fuel cells (DMFC), have been considered promising forpowering automobiles, homes, and portable electronics. Their successfulcommercialization is, however, very much dependent on the activity anddurability of their electrocatalysts. At present, all pre-commercial lowtemperature fuel cells use supported Pt and Pt alloys as theirelectrocatalysts. The critical properties to consider when choosing anelectrocatalyst support include its electrical conductivity, surfacearea, macro-morphology, microstructure, corrosion resistance, and cost.Carbon black (CB), such as Vulcan XC-72, has been the most widely usedelectrocatalyst support because of its reasonable balance amongelectronic conductivity, surface area, and cost. Recently, manynanostructured carbon materials with graphitic structure, such asnanotubes (CNTs), nanofibers (CNF) nanocoils, nanoarrays and nanoporoushollow spheres, have been studied. Among them, CNTs are of particularinterest due to their unique electronic and micro and macro structuralcharacteristics. CNTs have also been shown to be morecorrosion-resistant than CB under simulated fuel cell operationconditions.

Among the two variants of low temperature fuel cells, DMFCs have beenattracting great attention for powering small devices, such as laptopcomputers, cell phones, and personal digital assistants, because oftheir high energy density, ease of handling liquid fuel, and lowoperating temperature. However, the slow electrokinetics of the anodereaction—a methanol oxidation reaction—is still a key problem to thecommercialization of DMFCs. Normally, expensive noble metal alloys,typically Pt—Ru, with a high electrode metal loading (e.g., >2.0 mg/cm²)are employed in order to offer a reasonable fuel cell performance (e.g.,80 mW/cm² at cell temperature of 90° C. and O₂ pressure of twoatmospheres). It has long been desired for a high performance anodecatalyst to be developed so that the electrode metal loading and thusthe cost of DMFCs can be reduced.

Some early investigations have found that, by simply replacing CB withCNTs in the conventional ink-paste electrode fabrication method,superior DMFC performance can be obtained. For example, a DMFC singlecell with cup-stacked CNTs supported Pt—Ru anode catalyst showed nearlythree times the maximum power density of a DMFC with CB (Vulcan XC-72)supported Pt—Ru anode catalyst, and it was suggested that CNTs canprovide better charge and mass transfer.

Several types of carbon nanotubes may be used as electrocatalyticsupports for low temperature fuel cells, for example, single-walledcarbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) anddouble-walled carbon nanotubes (DWNTs). SWNTs can have a large surfacearea (e.g., 500-1000 m²/g) due to their small diameter (e.g., one nm),which is a favorable property as catalysts support. However, theynormally contain a significant amount (e.g., two-thirds) ofsemiconducting tubes, which are poor electron conductors and thus areexpected to be a poor electrocatalytic support. MWNTs are highlyconducting, but they have limited surface area (e.g., 100-200 m²/g) dueto their large diameter (e.g., forty nm). It was recently shown thatmost DWNTs are conducting tubes and that they can have high surfaceareas (e.g., 500-1000 m²/g). Thus a natural and logical choice for anelectrocatalyst support is DWNTs.

Accordingly, there is a need for, and what was heretofore unavailable,an improved membrane electrode assembly incorporating filtered and/ororiented carbon nanomaterials.

SUMMARY OF THE INVENTION

The present invention is directed to catalysts, electrodes and membraneelectrode assembly suitable for use in proton exchange membrane fuelcells (PEMFC) and direct methanol fuel cells (DMFC).

This invention provides a proton exchange membrane fuel cell withnanostructured components, in particular, the electrodes. Thenanostructured fuel cell has a higher precious metal utilization rate atthe electrodes, higher power density (kW/volume and kW/mass), and lowercost. The nanostructured fuel cells are not only attractive forstationary and mobile applications, but also for use as a compact powersupply for microelectronics such as laptops, cell phones and otherelectronic gadgets. In accordance with one embodiments of the presentinvention, aligned (oriented) arrays of carbon nanotubes are used assupport for catalyst. The aligned array of carbon nanotubes includessingle-walled, dual-walled, multi-walled, and cup-stacked carbonnanotubes. The precious metal in the catalyst (e.g., platinum andruthenium) is deposited using an ethylene glycol reduction method. Afterdepositing precious metal(s), a solubilized perfluorosulfonate ionomer(e.g., NAFION) may be incorporated into the spare space betweennanotubes to form a 4-phase boundary (gas, metal, proton conductor, andelectron conductor). By assembling the as-prepared electrodes with amembrane, gas diffusion layers and electron collectors, a membraneelectrode assembly is developed for incorporation into a fuel cell.

The common catalyst used in proton exchange membrane fuel cellsincorporates carbon black as a support structure (FIG. 4). However,carbon black does not provide sufficient simultaneous access of gas,proton, and electron. In addition, membrane electrode assemblies (MEA)using carbon black provide low platinum utilization (20-30%), a densecatalyst layer, low catalyst activity and are difficult mass transport.Conversely, a carbon nanotubes (CNT) or carbon nanofibers when orientedin a film as a catalyst support (FIG. 5) provides a high aspect ratiowith high platinum utilization having an easier balance of electron andproton conduction with easier mass transport, resulting from a uniqueinteraction of the carbon nanotubes with the metal particles and ahigher electrochemical activity. Such a carbon nanotube or nanofiberfilms further provide loose packing, orientation and hydrophobicity.Other favorable properties of carbon nanotube and nanofiber filmsinclude, but not limited to, high surface area, high electronconductivity and possible proton conductivity by known acidfunctionalization. Accordingly, a membrane electrode assembly usingcarbon nanotube or nanofiber films (FIG. 6) will have improvedperformance (FIGS. 11-14).

The present invention also includes a filtration method for preparingthe catalyst layer that is much simpler and faster than the traditionalink-paste method. This filtration method allows the catalyst layer to betransferred to the proton conductive membrane by hot pressing. Thefiltration method significantly advantageous over known sprayingprocesses by being much faster (e.g., seconds vs. minutes). It can alsolead to a partially oriented catalyzed carbon nanotube film. HigherPEMFC and DMFC performance may be achieved by the filtration methodmainly due to improved platinum utilization and the super-hydrophobicityof the carbon nanotubes. Accordingly, the electrode fabrication methodof the present invention is another important factor for obtaining ahigh fuel cell performance.

The PEMFC process of the present invention includes depositing platinumon carbon nanotubes to form the catalyst using ethylene glycol (EG) inthe reduction process. The platinum carbon nanotubes for the use in thecathode are formed by filtering the carbon nanotubes onto Nylon or othersuitable filter substrate. The oriented (aligned) carbon nanotubes aredried and then transferred onto a proton exchange membrane (PEM). Acathode diffusion layer is positioned on the outside of the platinumcarbon nanotubes. A membrane electrode assembly (MEA) is formed bysandwiching the anode catalyst (e.g., Pt/XC72) between the PEM and ananode diffusion layer that form the anode. The several layers of the MEAare hot pressed to form an integrated unit.

The DMFC process of the present invention includes depositingplatinum-ruthenium on carbon nanotubes to form the catalyst usingethylene glycol (EG) in the reduction process. The platinum-rutheniumcarbon nanotubes for the use in the anode are formed by adding(spraying) NAFION and filtering the carbon nanotubes onto Nylon or othersuitable filter substrate. The oriented (aligned) carbon nanotubes aredried and then transferred onto a proton exchange membrane (PEM). Ananode diffusion layer is positioned on the outside of theplatinum-ruthenium carbon nanotubes. A membrane electrode assembly (MEA)is formed by sandwiching the cathode catalyst (e.g., Pt/XC72) betweenthe PEM and a cathode diffusion layer that form the cathode. The severallayers of the MEA are hot pressed to form an integrated unit.

Other features and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic diagram of a membrane electrode assembly fora proton exchange membrane fuel cell.

FIG. 2 depicts a schematic of a membrane electrode assembly for a directmethanol fuel cell.

FIG. 3 is a typical fuel cell (H₂) polarization curve.

FIG. 4 is a schematic diagram of a catalyst having platinum disposed oncarbon black and NAFION polymer electrolyte.

FIG. 5 depicts a schematic of platinum containing carbon nanotubesoriented on a film.

FIG. 6 is an SEM of a membrane electrode assembly having a carbonnanotube based electrode.

FIG. 7 is a schematic representation of the method of preparing amembrane electrode assembly for a cathode for a PMFC.

FIG. 8 is a schematic of the method of preparing a membrane electrodeassembly for an anode for a DMFC.

FIG. 9A-9D are schematic representations of various carbon nanotubes.

FIGS. 10A-10C are TEMs of various carbon nanotubes.

FIG. 11 depicts PEMFC polarization curves for various platinum carbonnanotubes.

FIG. 12 depicts DMFC polarization curves for various platinum-ruthenium(PtRu) carbon nanotubes.

FIG. 13 depicts PEMFC polarization curves of aligned multi-walled carbonnanotubes having PTFE.

FIG. 14 depicts PEMFC I-V curves of oriented single-walled carbonnanotubes having PTFE.

FIGS. 15A-15C are SEMs of various surface contact angles.

FIG. 16 is a schematic representation of a rotating disk electrodesystem.

FIG. 17 is an evaluation of the electrocatalytic activity of oxygenreduction for platinum on various carbon supports.

FIG. 18 is an evaluation of the electrocatalytic activity of methanoloxidation for platinum-ruthenium on various carbon supports.

FIG. 19 depicts polarization curves for PtRu on carbon black anddual-walled carbon nanotubes.

FIG. 20A is a powder XRD pattern for platinum carbon nanotube catalysts.

FIG. 20B is a TEM of a platinum carbon nanotube catalysts.

FIG. 20C is a SEM of oriented carbon nanotubes containing platinum.

FIG. 21 is an elemental analysis plot of a platinum carbon nanotube.

FIGS. 22A-22F are TEMs of platinum-ruthenium multi-walled carbonnanotubes.

FIGS. 23A-23D are SEMs of platinum-ruthenium dual-walled nanotubes on aNAFION membrane and associated elemental analysis plots.

FIGS. 24A-24C are TEMs of cup-stacked carbon nanotubes.

FIG. 25 depicts polarization curves of cup-stacked carbon nanotubes.

FIG. 26 depicts polarization curves for platinum containing stacked cupnanotubes.

FIG. 27 is a TEM of a raw cup-stacked carbon nanotube.

FIG. 28 is a TEM of a cup-stacked carbon nanotube after oxidation.

FIGS. 29A and 29B are TEMs of platinum containing cup-stacked carbonnanotubes of about 22 weight percent and 15 weight percent platinum.

FIGS. 30A and 30B are TEMs of platinum containing cup-stacked carbonnanotubes having about thirty weight percent platinum.

FIG. 31 is an XRD plot for cup-stacked carbon nanotubes (nanofibers).

FIG. 32 depicts polarization curves for PEMFCs containing cup-stackedcarbon nanotubes.

FIG. 33 is a current density versus time (durability test) plot forcup-stacked carbon nanotubes.

FIG. 34 is a cell voltage versus current density (durability test) plotof cup-stacked carbon nanotubes.

FIG. 35 depicts polarization curves for platinum containing cup-stackedcarbon nanotubes.

FIG. 36 is a current versus potential plot of platinum containingcup-stacked carbon nanotubes compared to silver and silver chloridereference electrode.

FIG. 37 is a plot of the oxidation reduction reaction of platinumcontaining cup-stacked carbon nanotubes.

FIG. 38 is a Tafel plot for RDE data of cup-stacked carbon nanotubes.

DETAILED DESCRIPTION OF THE INVENTION

As shown in the drawings for purposes of illustration, the presentinvention is directed to membrane electrode assemblies (MEA) for protonexchange membrane fuel cells (PEMFC) and direct methanol fuel cell(DMFC). The electrodes (cathode for PEMFC and anode for DMFC) areconfigured (ethylene glycol deposition process) with platinum (Pt) orplatinum-ruthenium (PtRu) containing carbon nanotubes (CNT) that areoriented on a membrane. A filtering process (ethanol based) is used toalign (orient) the CNTs, such as single-walled, dual-walled,multi-walled and cup-stacked (stacked-cupped) onto a NAFION membrane toform the catalyst electrode. The MEA is produced by drying and pressingan anode and a cathode to each side of a proton exchange membrane,typically configured from a perfluorosulfonate membrane, such as NAFION(available from DuPont). Multiple MEAs are then combined in a stackhaving gas pathways to form an operable and testable fuel cell.

As shown in FIG. 1, a PEMFC 100 consists of an anode 110, a cathode 120,and a proton exchange membrane (PEM) 140. The assembly of these threecomponents is usually called a membrane electrode assembly (MEA). Ifpure hydrogen (H₂) 152 is used as fuel, hydrogen is oxidized in theanode and oxygen (O₂) 156 is reduced in the cathode. The protons 158 andelectrons 154 produced in the anode are transported to the cathodethrough the proton exchange membrane and external conductive circuithaving a load 150, respectively. Water (H₂O) is produced on the cathodeas a result of the combination of protons and oxygen. In order to makethe catalyst accessible by reactant gases, a hydrophobic diffusion layerconsisting of carbon particles and polytetrafluoroethylene (PTFE) isusually used to manage the water content around the catalyst layer.

Referring to FIG. 2, a direct methanol fuel cell 200 of the presentinvention includes an anode 210, cathode 220 and a polymer electrolytemembrane (PEM) 240 positioned between the anode and cathode. A methanol(CH₃OH) in water (H₂O) solution is introduced at the anode, whichreleases carbon dioxide (CO₂) during methanol oxidation catalyzed byplatinum (or other material) contained in the anode. Air or oxygen (O₂)is introduced at the cathode, and water is formed during oxygenreduction (catalyzed by platinum or other material) as protons (H+) moveacross the membrane. A load 250 connected across the anode and cathodecompletes the electric circuit formed by electrons (e−) released duringmethanol oxidation.

As shown in FIG. 3, a fuel cell polarization curve (I-V) 310 has anuppermost horizontal theoretical curve 310 with certain associated losesin efficiency. For example, there is a significant cathode kinetic loss320 and a somewhat smaller anode kinetic loss 350. In addition, there isan internal resistance loss 330 and a mass transport loss 340. PEMFCsalso exhibit a significant cathode over-potential loss, while DMFCsexhibit significant anode over-potential loss.

Referring to FIG. 4, the most commonly used electrode catalyst 400 isplatinum (Pt) 420, 425 supported on carbon fibers 410. One of thechallenges in the commercialization of PEMFCs and DMFCs is the high costof noble metals used as catalyst (e.g., platinum). Decreasing the amountof platinum used in a fuel cell electrode via the increase of theutilization efficiency of platinum has been one of the major concernsduring the past decade. To effectively utilize the platinum catalyst,the platinum should have simultaneous access to the gas, theelectron-conducting medium, and the proton-conducting medium. In thecatalyst layer of a platinum-based conventional fuel cell prepared bythe ink-process, the simultaneous access of the platinum particle 420 bythe electron-conducting medium and the proton-conducting medium isachieved via a skillful blending of platinum supporting carbon blackparticles 430 and the solubilized perfluorosulfonate ionomer (e.g.,NAFION) 450. The carbon particles conduct electrons and theperfluorosulfonate ionomer (e.g., NAFION) conduct protons.

Even with the most advanced conventional electrodes, there is still asignificant portion of platinum 425 that is isolated from the externalcircuit and/or the proton exchange membrane (PEM) 440, resulting in alow platinum utilization. For example, platinum utilization in currentcommercially offered prototype fuel cells remains very low (20-30%)although higher utilization has been achieved in laboratory devices.Efforts directed at improving the utilization efficiency of the platinumcatalyst have focused on finding the optimum material configurationswhile minimizing the platinum loading and satisfying the requirements ofgas access, proton access, and electronic continuity. In theconventional ink-process, a common problem has been that the necessaryaddition of the solubilized perfluorosulfonate ionomer (e.g., NAFIONfrom Ion Power, Inc.) for proton transport tends to isolate carbonparticles in the catalyst layer, leading to poor electron transport.

Due to their unique structural, mechanical, and electrical properties,carbon nanotubes may be used to replace traditional carbon powders inPEMFCs and have been demonstrated by making membrane electrodeassemblies (MEA) using carbon nanotube powders through a conventionalink process. The use of carbon nanotubes and the resulting guaranteedelectronic pathway eliminate the previously mentioned problem withconventional PEMFC strategies where the proton-conducting medium (e.g.,NAFION) would isolate the carbon particles in the electrode layer.Eliminating the isolation of the carbon particles supporting theelectrode layer improves the utilization rate of platinum.

Referring to FIG. 9A-9D, there are two categories of carbon nanotubes:single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). ASWNT (FIG. 9A) is a single graphene sheet rolled into a cylinder. A MWNT(FIG. 9C) is comprised of several coaxially arranged graphene sheetsrolled into a cylinder. One subset of MWNTs is dual-walled carbonnanotubes (DWNT) (FIG. 9B) that are formed from coaxially arrangedgraphene sheets rolled into a cylinder. According to theoreticalpredictions, SWNTs can be either metallic or semiconducting depending onthe tube diameter and helicity. The band gap is proportional to thereciprocal diameter, 1/d. For MWNTs, scanning tunneling spectroscopy(STS) measurements indicate that the conduction is mainly due to theouter shell, which are usually much larger than SWNTs. Therefore, MWNTshave a relatively high electrical conductivity. And, it is preferredthat MWNTs be the support for the platinum catalyst in PEMFCs because oftheir relatively high electrical conductivity and because current growthmethods for MWNTs are simpler than those for SWNTs.

Additional carbon nanomaterials for use in the electrocatalyst of thepresent invention include, but are not limited to, nanofibers andcup-stacked carbon nanotubes (CS-CNT), also referred to as “stacked-cup”carbon nanotubes. Nanofibers are essentially carbon nanomaterials placedend-to-end. Cup-stacked carbon nanotubes are an end-to-end collection ofopen-ended conical (frustum) cups, wherein each cup is a single layer ofgraphene like graphite (FIG. 9D).

Referring now to FIG. 7, the PEMFC process 700 of the present inventionincludes depositing platinum 735 on carbon nanotubes 710 to form thecatalyst 730 using ethylene glycol (EG) in the reduction process 720.The platinum carbon nanotubes for the use in the cathode 792 are formedby filtering 740 the carbon nanotubes onto Nylon or other suitablefilter substrate 750. The oriented (aligned) carbon nanotubes are driedand then transferred 760 onto a proton exchange membrane (PEM) 770. Acathode diffusion layer 796 is positioned on the outside of the platinumcarbon nanotubes. A membrane electrode assembly (MEA) 790 is formed bysandwiching the anode catalyst 795 (e.g., Pt/XC72) between the PEM andan anode diffusion layer 798 that form the anode 794. The MEA is hotpressed 780 to form an integrated unit.

The present invention includes a filtration process for forming membraneelectrode assembly films having carbon-based nanomaterials (for brevity,the process is described for carbon nanotubes, but is applicable tocarbon nanofibers and stacked-cupped nanotubes). Carbon nanotubes (CNTs)have been studied as an electrocatalyst support for proton-exchangemembrane fuel cells and direct methanol fuel cells. Some earlyinvestigations found that by using the normal paste method and simplyreplacing carbon black particles with disordered multi-walled CNTs asthe support for platinum (Pt) catalyst nanoparticles higher PEMFC andDMFC performances were achieved. There was also an effort to form asingle-walled carbon nanotube (SWNT) film via the electrophoretic methodfor studying the effect of SWNTs as a catalyst support in fuel cells.

It is believed that an oriented CNT film may offer much improved fuelcell performance over that of disordered CNTs because of its uniquemicrostructure. First, the electronic conductivity of CNTs is muchhigher along the tubes than across the tubes, and there is no energyloss when electrons transfer along the tubes. Second, higher gaspermeability is expected with the oriented CNT film. Third, an orientedfilm may also exhibit super-hydrophobicity, which can greatly facilitatewater removal within the electrode, thereby improving mass transport ina PEMFC or DMFC.

Aligned multi-walled carbon nanotube (MWNTs) film have been growndirectly on a carbon paper substrate by the chemical vapor deposition(CVD) method with subsequent deposition of platinum nanoparticles on theMWNTs to form the composite electrode. An advantage of the CVD approachis that the deposited platinum nanoparticles are almost guaranteed to bein electrical contact with the external circuit and much improvedplatinum utilization (e.g., sixty percent) has been achieved. With theaforementioned direct CVD method, however, it has been difficult thusfar to prepare platinum catalyst nanoparticles with a uniform smalldiameter (e.g., less than five nanometers) at high platinum loading(e.g., thirty wt %), two critical properties of a high-performanceelectrocatalyst. Also, it has been found that it is difficult to grow analigned CNT film directly on carbon paper by the CVD method because ofthe rough nature of the carbon paper. In addition, although the CVDprocess is well suited to the fabrication of micro fuel cells, theprocess is expensive to scale up if films with large areas (e.g.,hundreds of cm²) are desired.

In accordance with the present invention, a simple, versatile, andinexpensive filtration method for the preparation of an oriented Pt/CNTfilm for PEMFCs (FIG. 7) has been developed. In contrast to previousefforts where platinum nanoparticles were deposited after the CNT filmwas formed, the method of the present invention included depositing theplatinum (or Pt/Ru) catalyst nanoparticles on the CNTs before the filmformation (FIGS. 10A-10C). Filtration was used as a vehicle to form anoriented film of Pt/CNT on the NAFION membrane, which is then used asthe catalyst layer in the fabrication of the membrane electrode assembly(MEA) for PEMFC. The method is simple, convenient, and easy to scale up.This approach of the present invention takes advantage of all of theknown methods for depositing metal (e.g., platinum) nanoparticles on theCNTs with precise control on the particle size, distribution, andloading. It also offers a convenient method of preparing alloyelectrocatalysts, such as PtRu/CNT, PtNi/CNT, and PtSn/CNT.

In accordance with the method of the present invention, ethylene glycol(EG) was used as a reducing agent and solvent to prepare platinumsupported on the CNTs with a high metal loading (thirty weight percentplatinum). The Pt/CNT suspension was then filtered through 0.2 μm-porehydrophilic nylon filter paper. Because the nylon filter is hydrophilicand the CNTs are hydrophobic, the CNTs tend to “stand up” on the filterand self-assemble into an oriented film. The deposited film was thentransferred onto a perfluorosulfonate ionomer (NAFION) membrane bypressing the CNT-coated side of the filter onto the NAFION membrane.After the transfer, a five weight percent solubilized perfluorosulfonateionomer (NAFION) solution was sprayed onto the surface of the Pt/CNTfilm (1.0 mg dry NAFION/cm²) to complete the cathode preparation.Finally, the anode, the oriented Pt/CNT-film-coated NAFION membrane, anda gas diffusion layer were hot pressed to obtain a membrane electrodeassembly (MEA) with the oriented Pt/CNT film serving as the cathodecatalyst layer. A carbon black-based E-TEK electrode was used as theanode.

Suitable CNTs for use in the filtration method of the present inventioninclude commercial products synthesized by the CVD method (for example,multi-walled carbon nanotubes available from Material & ElectrochemicalResearch Corp.). To achieve a uniform platinum deposition on the outerwalls of the nanotubes, the CNTs were surface oxidized by a 2.0 MH₂SO₄/4.0 M HNO₃ mixture for six hours under refluxing conditions. Theoxidized and washed CNTs were then suspended in ethylene glycol solutionand mixed with hexachloroplatinic acid in ethylene glycol solution.After adjusting the pH of the solution to above thirteen with 1.0 M NaOHin ethylene glycol, the mixture was held at 140° C. for three hoursunder reflux conditions to reduce the platinum. After filtration,washing, and drying, a Pt/CNT catalyst metal loading of thirty weightpercent (wt %) was obtained.

For MEA preparation, both the anode and cathode include a carbon paperbacking layer, a gas diffusion layer, and a catalyst layer. Carbonpapers (available from Toray Corp.) treated with PTFE (ten wt % PTFE inthe cathode and anode) were employed as the backing layers. To preparethe diffusion layer, five weight percent PTFE solution and carbon blackXC-72 (available from Cabot Corp.) (weight ratio of carbon black to purePTFE—7:3) were mixed in ethanol by ultrasonication, and then the slurrywas sprayed onto the carbon paper as a gas diffusion layer with a carbon(C) loading of about four mg/cm² in both the anode and cathode. Theanode catalyst layer includes two layers. The first layer has a platinumelectrode area loading of 0.1 mg Pt/cm² (twenty wt % Pt/C, E-TEK). Itconsists of a seventy weight percent Pt/C catalyst and thirty weightpercent PTFE. The second layer also has a platinum electrode arealoading of 0.1 mg Pt/cm² (twenty wt % Pt/C, E-TEK), but it is composedof seventy wt % Pt/C catalyst and thirty weight percent NAFION (notPTFE). A five weight percent NAFION solution was finally sprayed ontothe surface of the anode (1.0 mg NAFION/cm²) to form a thin NAFIONlayer. The MEA was obtained by hot pressing the anode, an orientedPt/CNT-coated NAFION membrane, and a cathode gas diffusion/backing layerwith a pressure of 140 atmospheres (atm) at 135° C. for 1.5 minutes.

For comparison, two other cathode catalyst layers were prepared. One wasprepared by mixing twenty weight percent Pt/C E-TEK catalyst with PTFEsolution (weight ratio of Pt/C to pure PTFE—7:3) and depositing themixture onto the gas diffusion layer. The other was prepared by justspraying thirty weight percent Pt/CNT catalyst onto the gas diffusionlayer. Both cathode layers were sprayed with NAFION with a 1.0 mg/cm²loading. The platinum loading of the cathodes was 0.2 or 0.25 milligrams(mg) of Pt/cm². The single-cell performance of the five squarecentimeters (cm²) MEAs was conducted using an automated fuel cell teststation.

The x-ray diffraction (XRD) pattern (Bruker αXS, D8ADVANCE) of thePt/CNT sample is shown in FIG. 20A. The diffraction peak observed at23-27° can be attributed to the hexagonal graphite structure (002),which shows that CNTs have good electric conductivity. The platinumdiffraction peaks show that platinum has a face-centered cubic (fcc)crystal structure. From the isolated platinum (220) peak, the meanparticle size calculated with the Scherrer equation is 2.6 nanometers(nm). A typical transmission electron microscopy (TEM, Philips TECNI12)image of 30 wt % Pt/CNT (FIG. 20B) shows that spherical platinumnanoparticles with a narrow size distribution (2-5 nm and averaged at2.8 nm) have been deposited on the CNTs, which is consistent with theXRD analysis. Considering the fact that multiwalled CNTs have arelatively small surface area (100 m²/g) compared with that of carbonblack (237 m²/g), it is interesting that a small platinum particle sizecan still be obtained with this method at such a high metal loading (30wt %).

The scanning electron microscopy (SEM, Philips XL30-FEG) image of thecross section of the Pt/CNT-film-coated NAFION membrane (FIG. 20C) showsa thin film of about five μm thickness on the NAFION membrane. Most CNTsare standing up, although they are not fully vertical to the membrane.As those of ordinary skill in the art may appreciate a CNT film withenhanced orientation can be obtained with further optimization of thefiltration process. An elemental analysis is shown in FIG. 21.

The contact angles (CA) of the oriented Pt/CNT film (FIG. 15A) and thenon-oriented Pt/CNT film (FIG. 15B) (spraying Pt/CNT onto the cathodediffusion layer directly and without PTFE) are 151.7° and 134.2°,respectively. It is interesting that large contact angles are obtainedeven though these CNTs have been treated with a mixture of nitric acidand sulfuric acid before the platinum deposition. Apparently,hydrophilic functional groups (e.g., —COOH) were generated during theacid reflux process but may have been subsequently used for anchoringplatinum nanoparticles. It has been reported that an oriented pristineCNT film shows higher hydrophobicity than a non-oriented one. Thecontact angle measurements show that the Pt/CNT film prepared by thefiltration method is more hydrophobic probably because its orientationis better than that of the Pt/CNT film formed by the spaying method. Thecatalyst layer with seventy weight percent Pt/C and thirty weightpercent PTFE offers a contact angle of only 147.2° (FIG. 15C), which islower than that of the oriented Pt/CNT film without any PTFE. In a fuelcell electrode, PTFE is commonly added to serve as a binder and toprovide hydrophobicity for water management within the electrodestructure. However, the PTFE added to the electrode often covers theplatinum catalytic sites, thus lowering the activity of the platinumcatalyst. Furthermore, the use of PTFE requires sintering at hightemperatures (e.g., 340° C.), which tends to lead to the aggregation ofplatinum particles. Thus, the elimination of PTFE without sacrificinghydrophobicity and electrode integrity can be beneficial to themaximization of platinum utilization.

The polarization curves of the membrane electrode assembly (MEA) basedon oriented Pt/CNT, non-oriented Pt/CNT, and Pt/C (with thirty wt %PTFE) as the cathode catalyst layers are shown in FIGS. 11-14. As shownin FIG. 14, oriented Pt/CNT based MEA exhibits higher performance thanPt/C and non-oriented Pt/CNT based MEAs. Higher performance in theactivation-controlled region (low current density) may be attributed tothe enhanced specific activity of platinum due to the unique interactionof platinum and CNT. The elimination of the electronically insulatingPTFE and the possibly improved electron transfer through the orientedCNTs in the oriented Pt/CNT catalyst layer may help to improve platinumutilization and reduce the ohmic resistance of the electrode, which canprovide improved performance over the whole current density range. Atthe mass transfer controlled region (high current density), thesuper-hydrophobicity of the oriented Pt/CNT film can help to repel waterfrom the cathode, whereas the MEAs with non-oriented Pt/CNTs and Pt/Cshow lower performance that is mainly due to “flooding” of the electrodeand the resulting mass transport difficulty. Careful optimization of thestructure of the oriented CNT film (e.g., CNT alignment, film thickness,NAFION content, etc.) is likely to improve further the power density ofthe fuel cell.

The filtration method of the present invention demonstrates a simple,fast, inexpensive, and scalable filtration method of preparing apartially oriented super-hydrophobic CNT film via self-assembly of CNTsthat have been pre-catalyzed with small platinum nanoparticles (2.8 nm)at high metal loading (thirty wt %). When such a film is used as thecathode, higher PEMFC performance is achieved because of improvedplatinum utilization and mass transport. This method can be easilyextended to the preparation of oriented CNT supported bimetallicelectrocatalysts such as PtRu, PtNi, and PtSn, which are useful for bothPEMFCs and DMFCs.

Referring now to FIG. 8, the DMFC process 800 of the present inventionincludes depositing platinum-ruthenium 835 on carbon nanotubes 810 toform the catalyst 830 using ethylene glycol (EG) in the reductionprocess 820. The platinum-ruthenium carbon nanotubes for the use in theanode 892 are formed by adding (spraying) NAFION and filtering 840 thecarbon nanotubes onto Nylon or other suitable filter substrate 850. Theoriented (aligned) carbon nanotubes are dried and then transferred 860onto a proton exchange membrane (PEM) 870. An anode diffusion layer 896is positioned on the outside of the platinum-ruthenium carbon nanotubes.A membrane electrode assembly (MEA) 890 is formed by sandwiching thecathode catalyst 895 (e.g., Pt/XC72) between the PEM and a cathodediffusion layer 898 that form the cathode 894. The MEA is hot pressed880 to form an integrated unit.

Multi-walled carbon nanotubes (MWNTs) were purchased from the MER Corp.,which were produced by chemical vapor deposition (CVD). The MWNTs have adiameter of 80 to 120 nanometers (nm) and surface area of approximately100 m²/g. Dual-walled carbon nanotubes (DWNTs) were purchased fromNANOCYL S.A. and were also produced by CVD. The diameters of the DWNTsrange from 1.5 to 2 nm (bundled at 4 to 10 nm) and their surface area isabout 500 m²/g. The highly purified SWNTs were self-prepared by anarc-discharge method, and the bundled diameter is 4 to 10 nm and theirsurface area is also about 500 m²/g. All the CNTs in the experimentswere surface oxidized by a 4.0 N H₂SO₄—HNO₃ mixture for 6 hours underrefluxing condition.

Pt—Ru/CNT catalysts were prepared by an ethylene glycol (EG) reductionmethod. The preparation method is briefly described below using MWNTs asa typical example. The surface oxidized MWNTs (200 mg) were suspended inan EG solution and treated in an ultrasonic bath. Then an EG solution ofhexachloroplatinic acid and ruthenium chloride was added drop wise,under mechanically stirred conditions for 4 hours. A solution of 1.0 MNaOH in EG was added to adjust the pH of the synthesis solution to above13, and then the mixture was heated at 140° C. for 3 hours to reduce thePt—Ru completely. The entire EG solution has a deionized (DI) watercontent of five volume percent (vol %). A condenser was used in order tokeep the water content constant in the synthesis system and the wholepreparation process was conducted under flowing argon. After filtration(Whatman, Grade 1), washing, and drying in vacuum at 80° C. for 8 hours,the Pt—Ru/CNT catalyst was obtained. The filtrated solvent was clearwith a light yellow color and the weight calculation showed the Pt—Ruconversion was nearly 100% during the deposition process. We prepared 30wt % Pt—Ru/CNT catalyst for MWNTs, DWNTs, and SWNTs, and a 50 wt %Pt—Ru/DWNT catalyst. The 40 wt % Pt—Ru/C catalyst was kindly provided byE-TEK and used as a control sample.

Pt—Ru/CNT powders were characterized by X-ray diffraction (XRD) usingCu-Kα radiation with a Ni filter. The tube current was 40 mA with a tubevoltage of 40 kV. The 2θ regions between 20° to 85° were explored at ascan rate of 5°/min. Transmission electron microscopy (TEM) was carriedout on a PHILIPS S3000 operating at 300 keV. Scanning electronmicroscopy (SEM) was conducted on a PHILIPS XL30-FEG with an operatingvoltage of 10 or 15 kV.

The methanol oxidation reaction activity of the Pt—Ru/CNT catalyst wasconducted in a rotating disk electrode (RDE) setup using an Ag/AgClreference electrode, and a platinum wire counter electrode. The RDEworking electrode was prepared as follows. A mixture containing 7.6 mgPt—Ru/CNT catalyst and 2.0 mL ethanol was ultrasonically blended in aglass vessel for half an hour to obtain a homogeneous ink. Twenty μL ofink was spread with a micropipette on the surface of a glassy carbonelectrode (Pine Instrument, 5 mm in diameter) and dried for 10 minutes.The 10 μL NAFION solution (5 wt %) was dropped onto the surface of thethin catalytic layer. The electrolyte was 0.5 M H₂SO₄ solution. Thecyclic voltammetry (CV) tests were obtained after nitrogen bubbling for10 minutes. The scan range was from −0.3 V to 0.8 V and the scanincrement was ten mV/s.

For a DMFC membrane electrode assembly (MEA), both the anode and thecathode consist of a backing layer, a gas diffusion layer, and acatalyst layer. Teflonized (30 wt % TEFLON (PTFE) in the cathode and 10wt % in the anode) carbon papers (Toray Corp.) were employed as backinglayers in these electrodes. Then an ethanol suspension with 20 wt %Teflon and 80 wt % Vulcan XC-72 was agitated in an ultrasonic waterbath, and then spread on the cathode backing layer to prepare thecathode gas diffusion layers. The anode diffusion layer was similarlyprepared, but 20 wt % NAFION was used in place of 20 wt % Teflon.

For the anode catalyst layer fabrication, we used a modified filtrationmethod to make a compact Pt—Ru/CNT film. The schematic illustration ofthe fabrication process is described in FIG. 8. A Pt—Ru/CNT suspensionin ethanol with known solid loading was drawn through a 0.2-μm-porehydrophilic Nylon filter paper. To incorporate NAFION into the catalystlayer, the suspension was divided into five equal portions and thefiltration was carried out five times. After each filtration, a 20 wt %NAFION solution was spayed on the surface of the filtrated Pt—Ru/CNTsolid. Then the final deposited layer on the filter paper wastransferred onto NAFION membrane directly, by hot pressing the CNTcoated side of the filter onto the ionic membrane to produce a catalystcoated NAFION membrane (CCM).

The cathode catalyst layer includes two layers. The first layer is 1.0mg Pt/cm² (80 wt % Pt/C, E-TEK) mixed with 20 wt % PTFE, and the secondlayer is 1.0 mg Pt/cm² (80 wt % Pt/C, E-TEK) mixed with 15 wt % NAFION.A thin layer of 5 wt % NAFION solution was finally spread onto thesurface of cathode (1.0 mg NAFION/cm²).

The MEAs with an active electrode area of five cm² were obtained byhot-pressing a cathode, compact Pt—Ru/CNTs coated NAFION 115 membraneand an anode diffusion layer with a pressure of 50 kg/cm², at 135° C.for three minutes. For comparison, two conventional anode catalystlayers were prepared by spraying. The catalyst area loading were 2.0 mgPt—Ru/cm² and 0.5 mg Pt—Ru/cm² (40 wt % Pt—Ru/C, E-TEK). The catalystlayer also contained 15 wt % NAFION. The comparison MEAs were fabricatedby hot-pressing the anode, NAFION 115 and cathode according to samehot-press conditions as described above.

The MEAs were tested in a home-made automatic DMFC cell test instrumentwith a Scribner electronic load. The operation conditions are describedas follows: The oxygen operating pressure was 0.2 M Pa and the flow ratewas 200 mL/min, and the methanol concentration was 1 M and flow rate was2 mL/min. The temperature of cell and cathode humidifier was 90° C. and75° C., respectively.

The XRD patterns for the Pt—Ru/CNT samples were observed. Thediffraction peak at 23-27° observed is attributed to the hexagonalgraphite structure, which can reflect the graphite degree of a carbonmaterial. Among all of the samples, MWNTs have the highest diffractionpeak, indicating that MWNTs have the highest graphite degree and likelythe best electrical conductivity. SWNTs, DWNTs, and CB appear to havesimilar graphite degree. The analysis of other diffraction peaks showsthat all catalysts, Pt—Ru/CNT and Pt—Ru/C, have a Pt face centered cubic(fcc) crystal structure and Pt and Ru did not form alloys. No Ru peakwas detected, suggesting that Ru exists in an amorphous form. From theisolated Pt (220) peak, the mean particle size was about 2.0 nm,calculated with the Scherrer forn method, very small Pt—Ru nanoparticlescan be produced. The particle size does not depend on the support typebecause the Pt—Ru colloidal nanoparticles were produced before they weredeposited onto the support.

Typical TEM images of the Pt—Ru/MWNT, Pt—Ru/SWNT, Pt, Ru/C andPt—Ru/DWNT catalysts are shown in FIGS. 22A-22F. Overall, small Pt—Runanoparticles with diameter from two to three nm and tightly groupeddistributions are uniformly dispersed onto CNTs, even at high metalloading (i.e., 50 wt % Pt—Ru/DWNT). This is consismula for all thePt—Ru/CNT and Pt—Ru/C samples. This suggests that by using the EGreductiotent with the XRD results. Minor aggregation of nanoparticles onMWNTs is observed, and this is attributed to the lower surface area ofthe MWNTs (˜100 m²/g) (FIG. 22A). By contrast, aggregations ofnanoparticles were not observed on SWNTs, CB, or DWNTs (FIGS. 22B, 22C,22D, 22E, and 22F), most likely because SWNTs and DWNTs have highsurface areas (˜500 m²/g).

Referring now to FIG. 18, the Pt—Ru/CNT and Pt—Ru/C catalysts weretested for their methanol oxidation reaction (MOR) activity in ahalf-cell configuration using an RDE setup (FIG. 16). RDE tests canprovide the specific activity of the Pt—Ru catalyst in a well-controlledenvironment without mass transportation effects. Although the startingoxidation potential for these Pt—Ru catalysts were the same at about 0.1V, the oxidation peak for Pt—Ru/DWNT is about 0.037 mA/cm²Pt, which ismuch higher than the other samples (i.e., 0.019 mA/cm²Pt for Pt—Ru/MWNT,0.017 mA/cm²Pt for Pt—Ru/C, and 0.012 mA/cm²Pt for Pt—Ru/SWNT). In otherwords, the Pt—Ru/DWNT catalyst can generate higher currents, and thushave a higher specific activity than Pt—Ru/MWNT, Pt—Ru/CB, or Pt—Ru/SWNTcatalysts. The half-cell results suggest that Pt—Ru/DWNT catalysts wouldbe the best anode catalyst for a methanol fuel cell.

Before the fabrication of MEAs, the anode catalyst layer coatedmembranes were prepared. The cross-sectional SEM images and thecorresponding EDAX elemental analysis of the Pt—Ru/DWNT and Pt—Ru/DWNTcatalyst layer coated NAFION membranes are shown in FIGS. 23A-23D. Itwas noted that the MWNTs of the Pt—Ru/MWNT layers do not seem to havepreferred orientation as previously observed for a Pt/MWNT cathode layerfor PEMFC. It is believed that the orientation was a result of thehydrophobicity of the MWNTs and the hydrophilicity of the filter paper.Thus the unfavorable interaction forces the MWNTs to “stand up.” In thepresent study, NAFION was sprayed during the filtration process and isprobably responsible for the random orientation of the CNTs, becauseNAFION makes the CNT film more hydrophilic and the filter paper is alsohydrophilic. The Pt—Ru/MWNT catalyst layer is about 7-8 μm and thethickness for Pt—Ru/DWNT catalyst layer about 15-20 μm.

EDAX/SEM shows that platinum and ruthenium from the catalyst, carbonfrom support, and fluorine and sulfur from the NAFION are all present.The presence of fluorine and sulfur suggests that NAFION was mixed wellwithin Pt—Ru/CNT catalyst layer, and this is believed to be essential tofacilitate the mass transportation of the fuel, methanol. For elementalanalysis, five spots of the catalyst layer were examined from eachsample, and the calculated Pt:Ru ratio is 1:1.

The I-V curves of single DMFCs with different anode catalyst supportsand metal electrode loadings are shown in FIG. 19. The open circuitvoltage (OCV) for the Pt—Ru/DWNT catalyst (30 wt %, 0.5 mg/cm2) is 0.627V, which is 50 mV higher than that of Pt—Ru/C (30 wt %, 0.50 mg/cm²).Even when the electrode loading for the Pt—Ru/C was increased to 2.0mg/cm², the OCV was still about 0.620 V. The OCVs of Pt—Ru/MWNT andPt—Ru/SWNT based cells are 0.610 V and 0.583 V, respectively. From atheoretical point of view, the value of OCV can reflect the activity ofthe anode catalyst because the same cathode was used. The high OCV shownby the Pt—Ru/DWNT catalyst suggests that it has the highest MORactivity, which is consistent with the RDE results.

In the activation controlled region (i.e., at 0.55 V) the currentdensity for Pt—Ru/DWNT catalyst of 0.5 mg/cm² is 26 mA/cm², which ismuch higher than that of Pt—Ru/C catalyst with the same electrodeloading (0.5 mg/cm², 1.5 mA/cm2) and even much higher when the electrodemetal loading is four times as high (2.0 mg/cm², 8.5 mA/cm²).

The remarkably high MOR activity of Pt—Ru/DWNT is not fully understood.Some of the possible favorable factors are its high electricalconductivity, high surface area, and small diameter. The small diameterof the DWNTs may have lead to a unique interaction between platinum andthe DWNTs that facilitates charge transfer from platinum to the tubes,and thus increasing the specific activity of platinum. The bundlemorphology of the DWNTs may also help the uniform dispersion andretention of NAFION—further improving platinum mass activity.

From the overall I-V curves, Pt—Ru/DWNT catalyst (0.5 mg/cm²)outperforms Pt—Ru/MWNT at low current density regions, but it looses itsadvantage at the high current density regions. This is probablyattributed to the fact that the Pt—Ru/DWNT layer is thicker thanPt—Ru/MWNT layer (15-20 μm vs. 7-8 μm). In addition, it is also foundthat Pt—Ru/SWNT catalyst shows the lowest DMFC performance among allDMFC single cells, which is most likely because, that the SWNTs areusually a mixture of electrically conducting and semiconducting tubes.

To further improve the overall performance, a Pt—Ru/DWNT catalyst withhigh metal loading (50 wt %) was prepared and used in an MEA with lowelectrode metal loading of 0.34 mg/cm². Its DMFC I-V curve is also shownin FIG. 19. This MEA showed a superior DMFC performance at the wholecurrent density range and had the highest power density of 131 mW/cm².This is obviously attributed to its higher MOR, specific activity, andthinner catalyst layer. It is exciting to note that the DWNT supportedPt—Ru anode catalyst could offer a 63% enhancement of the DMFC's highestperformance when compared with carbon black supported anode catalystwith a 5.9 times more Pt—Ru noble metal.

The methods described regarding single-walled, dual-walled andmulti-walled carbon nanotubes may also be applied to cup-stacked carbonnanotubes. Various TEMs, SEMs, polarization curves and other relateddata are shown in FIGS. 24-38 for forming an electrocatalyst withcup-stacked carbon nanotubes.

EXAMPLES

The following examples are provided to illustrate the embodiments of thepresent invention. They are not intended to limit the scope of thisdisclosure to the embodiments exemplified therein. All ranges for allparameters disclosed are inclusive of the range limits.

Example No. 1 Single-Walled Carbon Nanotubes Process A:

-   -   1) Weigh SWNTs 1.00 g, put it into flask (250 mL with 3 necks);    -   2) Add 120 mL 2M H₂SO₄+4M HNO₃ into the flask, stir it at 1000        rpm for 10 minutes (min) at room temperature;    -   3) Ultrasonicate it for another 20 min;    -   4) Put the flask in a mantle heater (1000 rpm), increasing        temperature to 110° C. for eight hours;    -   5) Dilute the solvent by adding another 200 mL DI water in the        above-mentioned flask;    -   6) Filtrate it by normal filter paper and then by copious water        (1 L hot DI water, 85° C.) in batches;    -   7) Put the funnel with the filter paper and filtrate cake into a        convection oven at 110° C. for eight hours;    -   8) After drying, put the support into a glass bottle for later        use.

Process B:

-   -   1) Weigh SWNTs 1.00 g, put it into the flask (500 mL with 3        necks);    -   2) Add 150 mL EG into the flask and stir it for 10 min;    -   3) Ultrasonicate it for another 20 min (suspension becomes ink);    -   4) Put the flask in a heating mantle (hot plate/stirrer) and set        the stirring rate of 1000 rpm (the following procedures are all        under stirring);    -   5) Add 0.75 g H₂PtCl₆.x H₂O (37.5% Pt) and 0.29 g RuCl₃ (48.3%        Ru) in 30 mL EG by dropping (using funnel);    -   6) Let N₂ go through the flask and stir for 30 min, the        following procedures are all under N₂ protection;    -   7) Add 22 mL 1.0 M NaOH by dropping (using funnel) into the        flask to make pH=13;    -   8) Stir for another 30 min;    -   9) Increase the heating mantle's temperature to 135° C. in 15        min;    -   10) Keep flask under refluxing at the temperature (135° C.) for        3 h with 1000 rpm;    -   11) Cool down the flask naturally, keep at room temperature for        one hour with 1000 rpm;    -   12) Add 200 mL DI water into the flask;    -   13) Add 10 mL 2 M HCl by dropping (using funnel) into the flask        to make the pH=3;    -   14) Keep Stirring for 1.0 hour;    -   15) Stop stirring and wait for 1.0 hour;    -   16) Filtrate the PtRu/SWNTs (30 wt % Pt—Ru) solid with a regular        filter paper (Grade No. 3), diameter 4 cm, wash it by 1 L hot DI        water at 85° C.;    -   17) Put the funnel with the filter paper and filtrate cake into        a vacuum oven at 85° C. for eight hours;    -   18) After drying, put the catalyst into a bottle for later use.

Process C:

-   -   1) Weigh PtRu/SWNTs 42 mg, put it into flask;    -   2) Add a drop of H₂O to wet PtRu/SWNTs;    -   3) Add 10 mL ethanol and 362 mg NAFION (5 wt %);    -   4) Ultrasonic treating for 20 min, in order to make it become        ink;    -   5) Put a normal filter paper first, and then put a Nylon paper        (2 cm in diameter) in the funnel, drop 1 mL ethanol to wet the        paper;    -   6) Add the ink into the funnel;    -   7) Filter the ink to get a black solid onto the Nylon paper;    -   8) Weigh 420 mg NAFION solution (5 wt %) in a 50 mL beaker and        dilute it by same volume of ethanol, ultrasonic for 5 min;    -   9) Spraying the NAFION solution to surface of PtRu/SWNTs        deposited onto the Nylon filter, and drying it naturally in the        hood for 20 mins;    -   10) Cut the filter paper with PtRu/SWNTs catalyst into 1 piece        of 2.0*2.0 cm²;    -   11) Hot-pressing an anode, NAFION 115 membrane and a piece of        filter paper with PtRu/SWNTs at 120° C., 0.15 metric tons for 15        seconds;    -   12) Carefully peel the filter paper off;    -   13) Put the PtRu/SWNTs coated NAFION 115 membrane in a plastic        bag for MEA preparation;    -   14) Hot pressing a cathode and the PtRu/SWNTs coated NAFION 115        membrane at 135° C., 1.2 metric tons for 1.5 min.

Example No. 2 Dual-Walled Carbon Nanotubes Process A:

-   -   1) Weigh DWNTs 1.00 g, put it into flask (250 mL with 3 necks);    -   2) Add 120 mL 2 M H₂SO₄+4 M HNO₃ into the flask, stir it at 1000        rpm for 10 min at room temperature;    -   3) Ultrasonicate it for another 20 min;    -   4) Put the flask in a mantle heater (1000 rpm), increasing        temperature to 110° C. for eight hours;    -   5) Dilute the solvent by adding another 200 mL DI water in the        above mentioned flask;    -   6) Filtrate it by normal filter paper and then by copious water        (1 L hot DI water, 85° C.) in batches;    -   7) Put the funnel with the filter paper and filtrate cake into a        convection oven at 110° C. for eight hours;    -   8) After drying, put the support into a glass vial for later        use.

Process B:

-   -   1) Weigh DWNTs 1.00 g, put it into the flask (500 mL with 3        necks);    -   2) Add 150 mL EG into the flask and stir it for 10 min;    -   3) Ultrasonicate it for another 20 min (suspension becomes ink);    -   4) Put the flask in a heating mantle (hot plate/stirrer) and set        the stirring rate of 1000 rpm (the following procedures are all        under stirring);    -   5) Add 0.75 g H₂PtCl₆.x H₂O (37.5% Pt) and 0.29 g RuCl₃ (48.3%        Ru) in 30 mL EG by dropping (using funnel);    -   6) Let N₂ go through the flask and stir for 30 min, the        following procedures are all under N₂ protection;    -   7) Add 22 mL 1.0 M NaOH by dropping (using funnel) into the        flask to make pH=13;    -   8) Stir for another 30 min;    -   9) Increase the heating mantle's temperature to 135° C. in 15        min;    -   10) Keep flask under refluxing at the temperature of 135° for        three hours with 1000 rpm;    -   11) Cool down the flask naturally, keep at room temperature for        one hour with 1000 rpm;    -   12) Add 200 mL DI water into the flask;    -   13) Add 10 mL 2 M HCl by dropping (using funnel) into the flask        to make the pH=3;    -   14) Keep Stirring for 1.0 hour;    -   15) Stop stirring and wait for 1.0 hour;    -   16) Filtrate the PtRu/DWNTs (30 wt % Pt—Ru) solid with a regular        filter paper (Grade No. 3) diameter 4 cm, wash it by 1 L hot DI        water (85° C.);    -   17) Put the funnel with the filter paper and filtrate cake into        a vacuum oven at 85° C. for eight hours;    -   18) After drying, put the catalyst into a bottle for later use.

Process C:

-   -   1) Weigh PtRu/DWNTs 21 mg, put it into flask;    -   2) Add a drop of H₂O to wet PtRu/DWNTs;    -   3) Add 5.5 mL ethanol and 180 mg NAFION (5 wt %);    -   4) Ultrasonic treating for 20 min, in order to make it become        ink;    -   5) Put a normal paper first, and then put a Nylon paper (4 cm in        diameter) in the funnel, drop 1 mL ethanol to wet the paper;    -   6) Add the ink into the funnel;    -   7) Weigh 123 mg NAFION (5 wt %), and dilute it with same volume        of ethanol and then put into airbrush;    -   8) Filter the ink to get a black solid onto the Nylon paper        gradually by 5 times, after one time, spray NAFION on the top of        the filtrate cake;    -   9) Weigh 210 mg NAFION solution (5 wt %) in a 50 mL beaker and        dilute by same volume of ethanol, ultrasonic for 5 min;    -   10) Spraying the NAFION solution to surface of PtRu/DWNTs        deposited onto the Nylon filter paper, and drying it naturally        in the hood for 20 mins;    -   11) Cut the filter paper with PtRu/DWNTs catalyst into 1 piece        of 2.0*2.0 cm²;    -   12) Hot-pressing an anode, NAFION 115 membrane and a piece of        filter paper with PtRu/DWNTs at 120° C., 0.15 metric tons for 15        seconds;    -   13) Carefully peel the filter paper off;    -   14) Put the PtRu/DWNTs coated NAFION 115 membrane in a plastic        bag for MEA preparation;    -   15) Hot pressing a cathode and the PtRu/DWNTs coated NAFION 115        membrane at 135° C., 1.2 metric tons for 1.5 min.

Example No. 3 Cup-Stacked Carbon Nanotubes Process A:

-   -   1) Weigh CS-CNTs 1.00 g, put it into flask (250 mL with 3        necks);    -   2) Add 120 mL 2M H₂SO₄+4M HNO₃ into the flask, stir it at 1000        rpm for 10 min at room temperature;    -   3) Ultrasonicate it for another 20 min;    -   4) Put the flask in a mantle heater (1000 rpm), increasing        temperature to 110° C. for eight hours;    -   5) Dilute the solvent by adding another 200 mL DI water in the        above mentioned flask;    -   6) Filtrate it by normal filter paper and then by copious water        (1 L hot DI water, 85° C.) in batches;    -   7) Put the funnel with the filter paper and filtrate cake into a        conventional oven at 110° C. for eight hours;    -   8) After drying, put the support into a glass bottle for later        use.

Process B:

-   -   1) Weigh CS-CNTs 1.00 g, put it into the flask (500 mL with 3        necks);    -   2) Add 150 mL EG into the flask and stir it for 10 min;    -   3) Ultrasonicate it for another 20 min (suspension becomes ink);    -   4) Put the flask in a heating mantle (hot plate/stirrer) and set        the stirring rate of 1000 rpm (the following procedures are all        under stirring);    -   5) Add 1.14 g H₂PtCl₆.x H₂O (37.5% Pt) in 30 mL EG by dropping        (using funnel);    -   6) Let N₂ go through the flask and stir for 30 min, the        following procedures are all under N₂ protection;    -   7) Add 22 mL 1.0 M NaOH by dropping (using funnel) into the        flask to make pH=13;    -   8) Stir for another 30 min;    -   9) Increase the heating mantle's temperature to 135° C. in 15        min;    -   10) Keep flask under refluxing at the temperature (135° C.) for        three hours with 1000 rpm;    -   11) Cool down the flask naturally, keep at room temperature for        one hour with 1000 rpm;    -   12) Add 200 mL DI water into the flask;    -   13) Add 10 mL 2 M HCl by dropping (using funnel) into the flask        to make the pH=3;    -   14) Keep Stirring for 1.0 hour;    -   15) Stop stirring and wait for 1.0 hour;    -   16) Filtrate the Pt/CS-CNTs (30 wt % Pt) solid with a regular        filter paper (Grade No. 3) diameter 4 cm, wash it by 1 L hot DI        water (85° C.);    -   17) Put the funnel with the filter paper and filtrate cake into        a vacuum oven at 85° C. for eight hours;    -   18) After drying, put the catalyst into a bottle for later use.

Process C:

-   -   1) Weigh Pt/CS-CNTs 153 mg, put it into flask;    -   2) Add a drop of H₂O to wet Pt/CS-CNTs;    -   3) Add 5.5 mL ethanol;    -   4) Ultrasonic treating for 20 min, in order to make it become        ink;    -   5) Put a normal paper before the special paper first, and then        put a Nylon paper (4 cm in diameter) in the funnel, drop 10 mL        ethanol to wet the paper;    -   6) Add the catalyst ink into the funnel;    -   7) Filter the ink to get a black solid onto the Nylon paper;    -   8) Weigh 1.53 g NAFION solution (5 wt %) in a 100 mL beaker and        dilute by a 100 mL of ethanol, ultrasonic for 5 min;    -   9) Spraying the NAFION solution to surface of Pt/CS-CNTs        deposited onto the Nylon filter, and drying it naturally in the        hood for 20 mins;    -   10) Cut the filter paper with Pt/CS-CNTs catalyst into 1 piece        of 2×2 cm²;    -   11) Hot-pressing an anode, NAFION 115 membrane and a piece of        filter paper with Pt/CS-CNTs at 120° C., 0.15 metric tons for 15        secs;    -   12) Carefully peel the filter paper off;    -   13) Put the Pt/CS-CNTs coated NAFION 115 membrane in a plastic        bag for MEA preparation;    -   14) Hot pressing a cathode and the Pt/CS-CNTs coated NAFION 115        membrane at 135° C., 1.2 metric tons for 1.5 min.

While particular forms of the invention have been illustrated anddescribed, it will also be apparent to those skilled in the art thatvarious modifications can be made without departing from the inventiveconcept. References to use of the invention with a membrane electrodeassembly and fuel cell are by way of example only, and the describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The present invention may be embodied in otherspecific forms without departing from its spirit or essentialcharacteristics. Accordingly, it is not intended that the invention belimited except by the appended claims.

1-20. (canceled)
 21. An electrode, comprising a catalyst layer coatedproton exchange membrane; wherein the catalyst layer has an orientedfilm of a platinum-deposited carbon-based material; wherein thecarbon-based material is selected from the group consisting ofdual-walled carbon nanotubes, nanofibers and cup-stacked carbonnanotubes; wherein the catalyst layer is prepared using an ethyleneglycol reduction method.
 22. The electrode of claim 21, wherein thecatalyst is formed by using ethylene glycol to deposit platinum onto thecarbon-based material.
 23. The electrode of claim 21, wherein thecatalyst is further formed by depositing ruthenium onto the carbon-basedmaterial.
 24. The electrode of claim 23, wherein the catalyst is furtherformed by depositing solubilized perfluorosulfonate ionomer onto thecarbon-based material.
 25. A membrane electrode assembly for a fuel cellformed according to the following steps: forming a first catalyst bydepositing platinum onto a carbon-based material; filtering the firstcatalyst onto a substrate to form a filtered first catalyst having anoriented film of a platinum-deposited carbon-based material; providing afree standing proton exchange membrane; transferring the filtered firstcatalyst having an oriented film of a platinum-deposited carbon-basedmaterial onto a first side of the proton exchange membrane to form acatalyst layer coated membrane; placing a second catalyst coated secondgas diffusion layer adjacent to a second side of the proton exchangemembrane; placing a first gas diffusion layer adjacent the firstcatalyst layer on the first side of the proton exchange membrane; andusing heat and pressure to seal the first gas diffusion layer, the firstcatalyst layer coated membrane, the second catalyst coated second gasdiffusion layer together to produce the membrane electrode assembly.